Radar devices are being used with increasing frequency in vehicles to perform functions such as detecting objects in the vehicle vicinity and controlling vehicle equipment. This allows, for example, driver assistance systems to be provided which utilize data from the radar device to support the driver in braking or changing lanes.
The design of the frequency response curve is crucial to the target detection performed by a 24 GHz radar device that emits transmission signals in the 24 GHz range, for example. Usually, the transmission frequency of the transmission signal scans a specified bandwidth of 200 MHz using a roughly linear type of what are called chirps, which are frequency ramps with increasing transmission frequency. In modern radar devices, the gradient of these chirps is relatively low. Therefore, the time required for modern radar devices to scan 200 MHz is approximately 40 ms.
However, the general trend is clearly in the direction of substantially higher gradients. The reduction of the duration of a chirp to about 75 μs along with a higher scanning rate of the order of 1 MHz substantially improve target detection. Stringent requirements must be placed on the tuning voltage curve of the 24 GHz VCO (voltage-controlled oscillator) in order to achieve a linear transmission frequency curve in the presence of such high chirp gradients.
In the current state of technology, PLLs (phase-locked loops) are used to generate tuning voltage. As such, they combine with the VCO to create a closed control loop. However, this phase-locked loop entails extra costs that ideally should be avoided.